The present invention relates to scanning tunneling microscope (STM) emission spectroscopy, and more particularly to a light collecting device for scanning tunneling microscope emission spectroscopy which can collect light with high efficiency, which is a particularly important factor in measurement of a sample having a small emission intensity.
Conventionally, the following light collection devices using optical fibers have been used for STM emission spectroscopy.
(1) An optical (glass) fiber is extended along a single direction to the vicinity of a STM probe in order to collect light emitted from the STM probe and/or a sample [C. Thirstrup, M. Sakurai, K. Stokbro, and M. Aono, Phys. Rev. Lett. 82, 1241 (1999)].
(2) A plastic fiber [having a large numerical aperture (NA)] is extended along a single direction to the vicinity of a STM probe in order to collect light emitted from the STM probe and/or a sample. [M. J. Gallagher, S. Howells, L. Yi, T. Chen, and D. Sarid, Surf. Sci. 278, 270 (1992)].
(3) An optical fiber having a sharpened chip and coated with a transparent electrode is used as an STM probe, which is also used to collect light [T. Murashita and M. Tanimoto, Jpn. J. Appl. Phys. 34, 4398 (1995)].
In general, since quantum efficiency of STM emission is extremely low (about 10xe2x88x924 or less), it is not easy to measure emitted light while maintaining the high spatial resolution of an STM (i.e., without increasing tunnel current and bias voltage). Therefore, realization of high light collection efficiency is an important factor for STM emission spectrophotometry.
However, the above-described prior art technique (1), in which an optical fiber is extended along a single direction to the vicinity of a STM probe, involves a problem in that the light-collection solid angle is restricted to a value determined by the NA (about 0.2) of the optical fiber, and a light-collection solid angle greater than the value cannot be obtained.
The above-described prior art technique (2), in which a plastic fiber having a larger NA is used to obtain a larger light-collection solid angle, involves a problem in that, since the plastic fiber does not has heat resistance, use under a super-high vacuum is difficult. (In order to attain super-high vacuum, in general, baking must be performed at a temperature of about 150xc2x0. However, existing plastic optical fibers do not have resistance to heat of that temperature.)
The impossibility of performing experiments under super vacuum results in impossibility of using STM emission spectroscopy for evaluation of a clean surface and thus greatly narrows the range of applications of the STM emission spectroscopy.
The above-described prior art technique (3) involves a problem in that the necessity of machining the probe restricts the types of usable probe materials, which are important for STM measurement.
An object of the present invention is to solve the above-described problems and to provide a light collecting device for scanning tunneling microscope emission spectroscopy, which realizes higher light-collection efficiency under vacuum without sacrificing the high spatial resolution of an STM.
In order to achieve the above-described object, the present invention provides the following.
[1] A light collecting device for scanning tunneling microscope emission spectroscopy, characterized by comprising a scanning tunneling microscope; and a plurality of optical fibers radially disposed around the tip of a probe of the scanning tunneling microscope in order to collect light which is emitted upon operation of the scanning tunneling microscope, the optical fibers being disposed in such a manner that the distance between the optical fibers and the tip of the probe becomes a distance corresponding to a focal distance.
[2] A light collecting device for scanning tunneling microscope emission spectroscopy according to [1] above, characterized in that the optical fiber is made of glass.